Mobile communication systems allow mobility for users thereof. A mobile communication system may be provided by a cellular or otherwise wireless communication system using radio frequencies for sending voice or data between stations, such as user equipment (UE) (e.g. mobile stations, MS) and base transceiver stations (BTS), also called base stations. A base station system commonly comprises an antenna connected with appropriate electronics and control devices and so on. The radio frequencies may be divided in channels including traffic channels for voice and data transfer and signal channels for transferring control signals for the purposes of setting up a call or controlling an ongoing connection. Examples of cellular mobile communication systems include standards such as the GSM (Global System for Mobile communication) or various GSM based systems, such as GPRS (General Packet Radio Service), third generation standards, such as the UMTS (Universal Mobile Telecommunication System) and other communication systems that are based on the Wideband Code Division Multiple Access (WCDMA). In addition, local systems may be used to provide mobility. For example, a user may be provided wireless communications by means of systems that are based on any appropriate wireless local area network (WLAN) standards. A mobile user may access the mobile network, for example, by means of a user equipment, such as a personal computer (PC), personal digital assistant (PDA), mobile station (MS) and so on.
Various services can be provided for a user of a mobile user equipment by means of a communication system. Recent development in the field of mobile user equipment has lead to arrangements wherein information about the location of a mobile user equipment is determined and utilized when providing services for the user equipment or other parties. Such services are sometimes called location sensitive services. Location information may be used for various purposes, such as for location of a mobile telephone that has made an emergency call, for locating vehicles or mobile subscribers for commercial purposes and so on.
Information about the location of a mobile user equipment may be defined relative to the location of the base station. This is based on the assumption that a user equipment in communication with a base station must be located within the coverage area of that base station. The geographical location of the base station is commonly known, and therefore it is possible to conclude the location of the user equipment based on the identity of the base station.
Location based services are expected to become very important in relation to future mobile communication systems. As such, the need to accurately determine the position of mobile user equipment may increase and be based on commercial and security aspects.
A base station system having a plurality of remote radio units for transmitting and receiving at least one carrier signal has also been proposed. Such distributed radio units are referred to in the following as remote radio units, but may as well be called as remote heads, radio units, remote units and radio heads, or the like. Non-limiting examples of such distributed systems include an advanced indoor radio (AIR) and a distributed antenna system (DAS). For example, distributed wideband code division multiple access (WCDMA) technique may be used in channel multiplexing in such a distributed system. Signals transmitted from the remote radio units may not include an identification of the remote radio unit, but only an identification of the associated base station. Therefore, it may not be possible to determine from which of the remote radio units a signal has been received and thus which remote radio unit is closest to the mobile user. As a base station provided with remote radio units may cover a large area, the accuracy of the location information obtainable for a user equipment relative to the base station may in certain cases be insufficient, for example for commercial or emergency needs.
The accuracy of the location services may be improved by various manners from that of one cell. For example, location services may employ information of measurements regarding time of arrival, signal strength and so on. However, even with these it may be important to be able to tell precisely the radio unit which received signals from the target mobile user equipment.
Therefore, there is a need for improved system for identifying the remote radio unit closest to a mobile unit. It may be desired to achieve increased location accuracy for example inside a building, in particular when distributed base station systems are concerned.